title: The role of colonization pressure in nosocomial transmission of methicillin-resistant Staphylococcus aureus.
abstract: BACKGROUND
  Colonized or infected patients are a major reservoir for patient-to-patient transmission of methicillin-resistant Staphylococcus aureus (MRSA) in hospitals. Despite attempted adherence to recommended infection prevention and control procedures, a general medicine unit in our hospital continued to experience ongoing transmission of MRSA. The role that colonization pressure (CP) plays in nosocomial transmission of MRSA on a general medicine unit was assessed, and a threshold CP above which additional IP&C practices should be implemented was proposed.
  
  
  METHODS
  From January 2005 to December 2006, all patients admitted to a 36-bed general medicine unit were screened on admission for MRSA. Monthly MRSA nosocomial incidence (new nosocomial cases x 1000/susceptible patient-days) and CP (number of MRSA patient-days x 100/total patient-days) were calculated. The relative risk (RR) of MRSA transmission above and below the median CP with 95% confidence interval was calculated.
  
  
  RESULTS
  Twenty-one cases of nosocomially acquired MRSA were detected during the study period, with transmission occurring in 8 separate months. The median CP during the 2 years was 6.7%. The RR of MRSA acquisition increased as CP increased above the median (RR, 7.6; 95% CI: 1.1-52.6; P = .008). MRSA outbreaks were declared on 2 separate occasions, and, in each, the CP for the preceding month was greater than the median value of 6.7%.
  
  
  CONCLUSIONS
  CP has a significant effect on the subsequent transmission of MRSA on a general medicine unit. Ongoing monitoring of CP provides the opportunity for early implementation of enhanced infection prevention and control practices and can potentially decrease nosocomial transmission of MRSA and prevent outbreaks.
authors.affiliation: Department of Infection Prevention and Control, Sunnybrook Health Sciences Centre, ON, Canada. Victoria.Williams@sunnybrook.ca
authors.forename: Victoria R
authors.initials: VR
authors.lastname: Williams
authors.forename: Sandra
authors.initials: S
authors.lastname: Callery
authors.forename: Mary
authors.initials: M
authors.lastname: Vearncombe
authors.forename: Andrew E
authors.initials: AE
authors.lastname: Simor
mesh_terms: D002353:Carrier State / Q000453:epidemiology / Q000382:microbiology / Q000635:transmission*; D002408:Catheters, Indwelling / Q000382:microbiology; D003428:Cross Infection / Q000453:epidemiology / Q000382:microbiology / Q000635:transmission*; D016172:DNA Fingerprinting; D004196:Disease Outbreaks; D016521:Electrophoresis, Gel, Pulsed-Field; D006761:Hospitals; D006801:Humans; D055624:Methicillin-Resistant Staphylococcus aureus / Q000145:classification / Q000235:genetics / Q000302:isolation & purification*; D009666:Nose / Q000382:microbiology; D010502:Perineum / Q000382:microbiology; D015995:Prevalence; D013203:Staphylococcal Infections / Q000453:epidemiology / Q000635:transmission*; D014947:Wounds and Injuries / Q000382:microbiology